Nitrogen oxide emissions, principally nitrogen dioxide (NO.sub.2) and nitric oxide (NO) and referred to collectively as NOx, are acutely toxic air pollutants. Recent research findings link NOx to a broad range of air pollution problems, including acid deposition, the atmospheric production of photochemical ozone (the brownish haze in the air commonly called smog), health-threatening nitrate particles that also limit visibility, and the formation of toxic nitrogen compounds, such as nitrosamines. Annually, in the U.S. alone, over 20 million tons of NOx are emitted into the atmosphere. Carbon monoxide (CO) is also an acutely toxic air pollutant. It is the most commonly occurring air pollutant and can often be lethal. CO emissions to the atmosphere exceed all other pollutants combined. Major sources of these emissions are stationary sources, such as boilers, gas turbines, internal combustion engines, diesel engines, and incinerators, and mobile sources, such as automobiles.
In recent years, there has been an increased awareness of these emissions. As a result, the U.S. Environmental Protection Agency and others have promulgated and proposed standards to define the limits of NOx and CO permitted from various sources.
Three approaches can be taken to reduce NOx and/or CO emissions. These are: (1) Making changes before combustion; (2) Making modifications during combustion; and (3) Adding controls after combustion. Typical precombustion approaches are fuel switching, emulsifying the fuel with water, and denitrifying the fuel. Typical combustion modification techniques are changing stoichiometry, reducing temperature, and reducing residence time. Adding controls after combustion is generally referred to as flue-gas treatment.
NOx reduction during combustion has been employed since the early 1970's to obtain limited NOx emission reductions. It is the most common NOx emission reduction approach being used today to achieve moderate control. To obtain Higher levels of NOx reduction, it is generally necessary to employ a flue-gas treatment approach, or a combination of approaches.
Flue-gas treatment processes are of two types, dry processes and wet processes. Some processes are designed for the simultaneous removal of NOx and SO.sub.2. Many of the flue-gas treatment processes for NOx have been developed in Japan, where NOx emissions limits are generally stricter than in the United States.
Dry flue-gas treatment processes are normally preferred over wet processes because (1) they usually involve less equipment, and (2) they generally produce less waste that requires disposal. Most dry processes, however, share one characteristic with wet processes: they are both very expensive. A number of dry processes are either commercially available or are well along in development. They range from catalytic and noncatalytic reduction to absorption processes and irradiation with electron beams.
Today, the most popular flue-gas treatment process for NOx by far, at least for stationary sources, is selective catalytic reduction (SCR). In SCR, ammonia is used as a reducing agent. In the process, NOx is reduced to N.sub.2 and H.sub.2 O by ammonia at 300.degree. to 400.degree. C. in the presence of a catalyst. Ammonia is an acceptable reducing agent for NOx in combustion gases because it selectively reacts with NOx while other reducing agents, such as H.sub.2, CO, and methane, readily react with O.sub.2 in the gases. The catalysts are normally precious metals, such as platinum, rhodium, palladium, ruthenium, osmium or iridium, or zeolites.
Major problems identified with present SCR systems include: (1) poisoning, masking, and thermal aging of the catalysts; (2) ammonia slip (some ammonia invariably passes through the catalyst bed unreacted and out with the flue gas); (3) difficult process control; (4) limited temperature range for satisfactory NOx conversion; and (5) high costs. Poisoning, masking, and thermal aging can destroy the catalyst's activity. Poisoning is the chemical reaction of components in the system with the catalyst itself. The reaction is often irreversible. Excessive amounts of sulfur, phosphorus, and certain metals, for example, can poison catalysts. Masking is caused by a gradual accumulation of non-combusted, solid material on the catalyst's surface. It prevents gases from contacting the catalyst's surface. Thermal aging is a sintering process. Precious metal catalysts are generally deposited as extremely fine particles on the surface of a carrier phase. With time, these fine particles migrate along the surface and fuse with other particles forming larger particles. In doing so, their total surface areas decrease, along with their activity.
Ammonia slip can be a serious problem. It can occur at ammonia:NOx ratios well below 1.0. An ammonia:NOx ratio of 1.0 is the amount of ammonia required stoichiometrically to react precisely with all the NOx present in a gas. Because it is generally impossible to achieve 100 percent reaction at a ratio of 1.0, to achieve near 100 percent conversion, systems must operate at higher ratios and subsequently they show major ammonia slip. To operate at 15 percent below a ratio of 1.0 (which is common practice) means that only enough ammonia is available to react with 85 percent of the NOx present.
Control systems for present SCR systems tend to be elaborate and costly, and generally they add significant pressure-drop penalties. Also, in most applications, the compositions and temperatures of the flue gas vary to a degree. For conventional SCR systems, as the NOx level in the gas changes, the amount of ammonia introduced into the flue gas stream must be adjusted. If these adjustments are not made, either major ammonia slip occurs or NOx reduction performance falls off. Also, because most NOx catalysts only perform satisfactorily within narrow temperature ranges, heat exchangers to remove heat and burners to add heat are often employed to bring the flue gas temperature into the desired range. Controls, therefore, are critical. The temperature, the ammonia level and the NOx level of the gas stream must be constantly monitored, and changes must be communicated to the gas heating and cooling systems and to ammonia-addition equipment so adjustments can be made accordingly.
Present SCR systems are costly because they require expensive catalysts, because they require expensive control systems, and because catalyst regeneration, where possible, is an expensive procedure (Acid treatment is a common regeneration procedure).
In U.S. Pat. No. 4,806,320, the Inventor, S. Nelson, describes a process in which expanded vermiculite, a low-cost mineral, is used in place of expensive, more conventional catalysts in an SCR process. The process, which involves the use of ammonia or methane in combination with vermiculite, performs well in reducing the NOx levels of flue gases.
In more recent SCR research work supported by the U.S. Air Force under SBIR Contract FO8635-88-C-0262, the Inventors made three surprising, unexpected discoveries: (1) That very attractive NOx reductions, up to 100 percent, can be obtained with the use of expanded vermiculite alone, with no need whatsoever for ammonia or methane gas injections; (2) That these high reductions can be obtained over a wide range of temperatures, 20.degree. to 600.degree. C.; and (3) That CO is reduced, in addition to NOx, during processing and that the CO reduction products are gaseous oxygen and solid carbon, the latter depositing for the most part on the vermiculite particles. Further, it was discovered that over time these carbon deposits reduced the NOx and CO conversion efficiencies, but that good conversion efficiencies could be recovered simply by heating the vermiculite catalyst for a short time in air or in flue gas supplemented with air at temperatures above 450.degree. C. When this was done, the carbon was burned off as carbon dioxide (CO.sub.2). In addition, it was found that two other materials, expanded perlite (a low-cost mineral similar to vermiculite) and borosilicate glass wool can be substituted for vermiculite resulting in NOx and CO reductions almost as good as those achieved with vermiculite. Further, it was found surprisingly that by heating expanded vermiculite in air at a temperature above 300.degree. C. just prior to its contact with a flue gas markedly improves both NOx and CO conversion efficiencies.
The ramifications of these discoveries are significant. The use of vermiculite alone, or alternately perlite or borosilicate glass wool, means that a very simple process can be used to reduce simultaneously the NOx and CO levels of flue gases. It means, because no gaseous ammonia is required for NOx reduction, the possibility of ammonia slip is totally eliminated, that the expensive controls required in conventional SCR systems are not needed, and that gas-temperature adjustment equipment may not be required.